The present invention relates to a hybrid battery electric drive using two different batteries having high energy density and high power density, respectively.
Specific power and specific energy of batteries are usually related in an inverse manner. A battery with high specific power normally has a low specific energy, and a battery with high specific energy a low specific power. A fuel cell battery, a zinc-air battery and a radioisotope generator are regarded as a battery with high specific energy, whereas a lead-acid battery, an alkaline battery and a condensor as a battery with high specific power. For brevity, a battery with high specific power to give higher power will be hereinafter referred to as a P-battery while a battery with high specific energy to give better range will be referred to as a E-battery.
Many hybrid battery systems have been developed to meet variable load in powering a propulsion motor or motors of an electric vehicle or submarine. One typical hybrid battery system of them uses a P-battery electrically paralled with a E-battery and is electrically arranged with a dc electric motor such that the E-battery produces the power to operate the dc electric motor and recharges the P-battery during light load operation, and both batteries feed power to the motor during heavy load operation. One problem of the hybrid battery system is that for enabling electric current to be fed from the E-battery to the P-battery when the recharge of the P-battery is demanded, the terminal voltage of the E-battery has to be higher than that of the P-battery either by increasing the terminal voltage of the E-battery or by lowering the terminal voltage of the P-battery. On present state of development of the batteries a fuel cell battery or zinc-air battery is feasible for the E-battery. Such a battery is constructed of a plurality of individual cells each developing a low voltage and being provided with an electrolyte recirculator for the elimination of water consumed by each cell and the cooling purpose. Thus if the terminal voltage of this battery is to be increased to make the hybrid battery system feasible, a number of cells have to be arranged one after another along with the corresponding number of electrolyte recirculators attached to the cells. The results are that the leak current through the recirculated electrolyte increases because of the increase in quantity of the electrolyte and that the construction becomes complicated. If, on the contrary, the terminal voltage of the P-battery is lowered, the quantity of electric current must be increased to obtain the same power output. Usually in an electric vehicle, the power to be produced by the P-battery occupies a greater proportion of all the power to operate a dc electric motor during the full load operation. The ratio of the power given by the E-battery to that by the P-battery in the hybrid battery system under full load operation of the electric vehicle is approximately 1:3. It will therefore be understood that the cost of circuit configuration increases because control elements having a large capacity to control large current from the P-battery are necessary and that the efficiency reduces because of the considerable power loss with the use of large current with low voltage.